1. Field of the Invention
The present invention relates to an electric power steering apparatus which generates a steering assist force by an electric motor.
2. Description of Related Arts
There is known an electric power steering apparatus of a so-called rack assist type, which is adapted to convert the output rotation of a steering assist electric motor into the axial movement of a steering shaft including a rack through a motion converting mechanism such as a ball screw mechanism.
In the electric power steering apparatus, a noise (rattle) is liable to occur due to backlash of the components of the ball screw mechanism.
In general, the backlash should be minimized within a range allowed by the accuracy of the machining of the components in order to minimize the noise. More specifically, a so-called matching assembly process is performed, in which components for the ball screw mechanism are classified according to deviation in dimensional accuracy, and components providing an acceptable combinational dimensional accuracy are assembled into the ball screw mechanism. However, the matching assembly process requires much labor, resulting in higher production costs.
An electric power steering apparatus having a support arrangement shown in FIGS. 5A, 5B and 5C is disclosed, for example, in Japanese Unexamined Patent Publication No. 10-194140 (1998).
Referring to FIG. 5A, a ball nut 91 of a ball screw mechanism is engaged with a first end portion 92a of a steering shaft 92 in the electric power steering apparatus. A pinion 93 coupled to a steering wheel (not shown) is meshed with a second end portion 92b of the steering shaft 92. A rack guide 94 including a biasing member for biasing the steering shaft 92 toward the pinion 93 is disposed between the ball nut 91 and the pinion 93. Referring to FIG. 5B, the steering shaft 92 is supported at three points by the ball nut 91, the pinion 93 and the rack guide 94.
On the other hand, an electric power steering apparatus having a support arrangement shown in FIGS. 6A, 6B and 6C is disclosed, for example, in Japanese Unexamined Patent Publication No. 10-194149 (1998).
Referring to FIG. 6A, the electric power steering apparatus has substantially the same construction as the electric power steering apparatus shown in FIG. 5A, except that a stopper 95 for limiting the warp of the second end portion 92b of the steering shaft 92 is additionally provided. Referring to FIG. 6B, the steering shaft 92 is supported at three points, under normal conditions, by the ball nut 91, the pinion 93 and the rack guide 94 in the same manner as in the electric power steering apparatus shown in FIG. 5B. When the second end portion 92b of the steering shaft 92 is warped opposite from the pinion 93 beyond a predetermined warpage, the second end portion 92b is supported by the stopper 95.
In the former and latter electric power steering apparatuses, the steering shaft 92 is disposed transversely of a motor vehicle. The pinion 93 is disposed forwardly (in a direction F) of the steering shaft 92 with respect to the vehicle, and the rack guide 94 is disposed rearwardly (in a direction R) of the steering shaft 92 with respect to the vehicle.
In the former and latter electric power steering apparatuses, the rack guide 94 disposed opposite from the pinion 93 with respect to the steering shaft 92 is located closer to an axially middle position (closer to the ball nut 91), so that a support span Y between the ball nut 91 and the rack guide 94 is relatively short. Thus, the rack guide 94 presses the steering shaft 92 against the ball nut 91 pivotally on a mesh point of the steering shaft 92 with the pinion 93, whereby the rattle of the ball screw mechanism is eliminated to reduce the noise.
However, where the steering shaft 92 is warped by a force received from a road surface with the first and second end portions 92a, 92b thereof displaced rearward (in the direction R) as shown in FIG. 5C in the former electric power steering apparatus, the steering shaft 92 is supported at two points by the ball nut 91 and the rack guide 94. Since the support span Y between the ball nut 91 and the rack guide 94 is short, there is a possibility that the support rigidity of the steering shaft 92 is insufficient.
Where the steering shaft 92 is warped by a force received from a road surface with the first and second end portions 92a, 92b thereof displaced rearward (in the direction R) as shown in FIG. 6C in the latter electric power steering apparatus, the second end portion 92b of the steering shaft 92 is not supported until the second end portion 92b is warped rearward (in the direction R) beyond the predetermined warpage. As a result, there is a possibility that the support rigidity of the steering shaft 92 is insufficient.